House Gardener of Highgarden is the extinct house of the old and famed Kings of the Reach. Their blazon was a green hand over a white field.
They died out during fighting the Targaryens the Wars of Conquest. The last king of the line, Mern IX, joined forces with King Loren Lannister of the Rock and faced Aegon and his sisters on the Field of Fire, were he was burned alive with many of his men. His stweard, Harlan Tyrell, surrendered Highgarden to Aegon and was named Lord of the Reach in exchange of his submission to the Iron Throne.
The first reference to their rule dates two thousand years before the events described in A Song of Ice and Fire, when they armed the inhabitants of the Shield Islands to protect the Reach from the attacks of the ironborn. That establishes a rule of at least one thousand seven hundred years.
The best known member of the house is the mythical king of the First Men Garth Greenhand, who according to legend wore a crown of vines and flowers and made the land bloom.
There were at least eleven more Garths on the house, since the money that was found at Rugen's cell at King's Landing had been minted by Garth XII.
Gyles the Third was a king that went to war against the Storm King, and was attacked by the King of the Rock while he was with his army at the east.
The last member of the line was King Mern IX, who died at the Field of Fire and with all proability had no offspring.
A lot of houses from the Reach can trace their descent back to Garth Greenhand, a fact that is often used to boast of a better claim than the Tyrells on the ovelrdship of the Reach (The Tyrells are descendants of the Gardeners only through the female line).
Among the houses with Garth as an ancestor there are the Florents, the Rowans, and the Oakhearts, and many others. Apparently Garth had many sons himself.